The Coalition is caught in a deepening row over how to tackle illegal
immigration amid concerns that a new Home Office campaign could undermine
community cohesion.

Vince Cable, the Business Secretary, has described a government campaign telling illegal immigrants to “go home or face arrest” as “stupid and offensive”.

He spoke out about his wider concerns over Conservative immigration policy ahead of the launch of his new initiative on Monday to attract tens of thousands more foreign students to this country.

There are also growing doubts over how accurate estimates of the number of legal immigrants in Britain are after a committee of MPs dismissed the official figures held by the Government as “little better than a guess”.

In his weekly column for The Daily Telegraph, Boris Johnson warns that those here illegally now have to be asked “bluntly” to leave Britain because there is “virtually no chance” they will be deported.

The Mayor of London denies it is “racist” to run an advertising campaign asking them to leave, claiming that sophisticated legal teams have been able to thwart attempts to remove illegal immigrants from the country.

He describes the Home Office as running a “de facto amnesty” that has led to thousands of illegal immigrants living and working in Britain without fear of deportation.

In recent weeks, Conservative ministers have piloted the billboard campaign in London telling illegal immigrants to “go home or face arrest”. It was hoped the scheme would be rolled out nationally.

However, the strategy has prompted outrage from the Liberal Democrats and community groups who claim it is divisive and heightens racial tensions. Even Nigel Farage, the leader of Ukip, voiced concern.

Mr Cable said it was already difficult to be an immigrant in Britain and made clear that the Lib Dems had not sanctioned the Home Office’s “pilot” scheme. He claimed that the Conservatives were “obsessing” about the total level of immigration, which was leading them to make poor policy decisions.

He will unveil a new higher education strategy designed to attract an extra 90,000 foreign students to Britain by 2018.

In his column, Mr Johnson describes the Home Office campaign as “uncompromising” and admits that the message “could have been more gently drafted”.

However, he goes on to suggest that extreme measures may be necessary.

“Illegal immigrants have every opportunity to make their case to remain in Britain, and we have courts full of eloquent Lefty lawyers… taking prodigious sums of taxpayers’ money to vindicate the human rights of their clients,” he says.

“Such is the ingenuity of these lawyers that all government strategies to deal with these illegals have so far failed. Indeed, we already have a de facto amnesty for all illegal immigrants who have been able to stay here for a long time.

“Ask the Home Office how many illegal immigrants have been deported, after being here for more than 10 years. The number is tiny.”

Mr Johnson concludes that the current situation is unfair for “hard working and otherwise law-abiding illegal immigrants” who have little chance of playing a meaningful role in society.

Previously Mr Johnson backed an official amnesty for illegal immigrants who had been in the UK for more than 10 years so that they could begin paying taxes, a proposal the Government has rejected.

He added: “It is certainly not racist to point out this absurdity, since illegals come from all ethnic groups. It is not anti-immigrant to point this out, since illegals make a nonsense of the efforts of other immigrants to do the right thing and secure leave to remain. One way or another illegals need to regularise their position.”

Mr Cable said that the giant billboards, which were driven around towed by white vans, created a false “sense of fear” that Britain has a “vast problem” with illegal immigration.

He said that the billboards, which have so far been piloted in six London boroughs, are “unlikely” to be rolled out nationwide.

He said: “It’s stupid, the whole idea that illegal immigrants have got a sophisticated grasp of English. It is offensive.

“It is designed apparently to create a sense of fear in the British population that we have a vast problem of illegal immigration. We have a problem, but it’s not a vast one and it’s got to be dealt with in a measured way.

“I wasn’t consulted, and I know my other Liberal Democrat colleagues haven’t been. I can’t see that I would be in favour of it [continuing]. I think it very unlikely that it will continue.” He also criticised David Cameron’s “obsession” with reducing net immigration to tens of thousands of people, saying it was based on “misleading” statistics and was “not good for the country”.

Mr Cameron has committed to cutting net immigration to less than 100,000 a year by 2015, from 252,000 in 2010.

However, the public administration select committee has criticised the official figures as “little better than a best guest” and suggested they were a “virtually meaningless”.

Mr Cable said: “It’s quite difficult being an illegal immigrant in Britain. You can’t work, you can’t have access to benefits. The idea that there is some vast hidden army of people is almost certainly completely wrong. There is this net immigration figure which the Conservatives are very preoccupied by. It’s very misleading. The largest number of people counted as immigrants are overseas students, who are not immigrants.

“They are visitors. Under UN classification they are regarded as immigrants but they’re good for the country. So obsessing about this net immigration number is not good for the country.”

Sadiq Khan, the shadow justice secretary, said the campaign was “short sighted”. He said: “I think this is David Cameron’s attempt to try to win over Ukip voters."